The only lesson that doesn’t carry over is the trigonometry. 3D printers don’t use trig.

Why would I get a Makelangelo when I can get a 3D printer? Plastic is maybe USD20/kg.

USD20 may not be much to you but it’s a lot for a low-income classroom with a $100/year budget. Add to that the risk of a young person burning or electrocuting themselves and the hidden cost of a 3D printer becomes really unattractive. If a Makelangelo messes up it wastes a sheet of paper and maybe a marker.

Can I use a pencil, chalk, paintbrush, or a ball point pen?

Not really. Pencils and chalk change shape as they are being used up. The machine would have to be frequently paused and adjusted. There’s a similar problem with paint. Ballpoint pens need a surprising amount of pressure to make the ball roll (which leaves ink behind).